Extended drain MOS (EDMOS) transistors are one of the main devices used in the embedded power market. Their structure is similar to conventional MOSFETs. EDMOS transistors comprise a region between the drain region and the channel region that is called the drift region and that is lowly or lightly doped. This drift region is used to get higher breakdown voltage than the one of the p-n junction between channel and drain.
Such a conventional EDMOS, which is similar to the one shown for example in US 2004/002196, transistor is schematically shown in FIG. 7. In FIG. 7A there is shown a NMOS transistor in a cross-sectional view along a longitudinal axis from source to drain. The EDMOS transistor 700 of FIG. 7A comprises a substrate 701 having different regions formed thereon. From the left to the right in FIG. 7A a source region 702 formed by a highly doped region (N++) and connected to a source region contact 703 is shown. Next a channel region 704 is formed by a p-well on top of which a control gate 705 is formed which has a control gate contact 706 and which is insulated from the channel region 704 by a gate insulating layer 707. Next a drift region 708 is formed in the substrate, which drift region is only lightly doped (N−). A highly doped region (N++) forms a drain region 709, wherein the drain region 709 is connected to a drain contact 710. An arrow 711 schematically illustrates the current flow. FIG. 7B shows a perspective view of the EDMOS transistor of FIG. 7A. For illustrative reasons a portion of the drain region 709 and the drift region 708 are cut away. Furthermore, the different regions are shown as simple cuboids.
Depending on the type of breakdown voltage wanted the drift region will be extended or not and/or its doping will be lowered or increased. So for low breakdown voltages the length of this region is relatively small and its doping relatively high. However, this leads to the fact that the contribution to the overall resistance of the transistor or device is relatively limited and the overall resistance is caused mainly by the channel resistance.